This invention relates to a driving device for an injection molding machine, and particularly to driving devices for an injection unit and a mold-clamping unit.
A ball screw shaft is used to drive various members along the axis of a molten resin extrusion screw of an injection molding machine. The ball screw comprises a threaded shaft formed with a thread groove on its outer periphery, a nut having a thread groove on its inner periphery and being screwed onto the threaded shaft, and balls received in the thread grooves of the shaft and the nut.
The threaded shafts of conventional ball screws are formed by induction-hardening steel (e.g. AISI4150) or by carburizing chrome-molybdenum steel (e.g. SCM415) to the hardness of about 58-62 in HRC. The nuts are formed by carburizing e.g. SCM415 to the hardness of about 58-62 in HRC. The balls are formed by hardening bearing steel (e.g. SUJ2) to the hardness of 60 in HRC or over.
Among the parts of a ball screw, the balls' life tend to expire first. This is because when the nut is moved by rotating the threaded shaft, the contact surface between the threaded shaft and the nut becomes oval, and the distances between the center of the threaded shaft and various points of the oval contact surface differ from one another, so that the balls purely roll only at two points of the oval contact surface and slip at the other area. As a result, the balls are damaged due to this differential slipping.
In the case of a ball screw, compared with rolling bearings, its shape makes it difficult to finish to groove shape accuracy and surface roughness comparable to inner and outer bearing rings. Thus, if bearing balls are used for the balls of a ball screw, the balls are liable to be damaged severely. Thus, the balls' life expires first.
Among ball screw type driving devices for injection molding machines, a ball screw used in a driving device used for an injection unit to axially advance and retract an extrusion screw for extruding molten resin, and a ball screw used in a driving device used for a mold-clamping unit for advancing and retracting a movable platen tend to be short in stroke from when a load begins to act until the ball screw stops, so that the balls tend to stop abruptly. Thus, a load nearly as heavy as an impact load acts on the balls.
When the balls' life expires, the life of the entire ball screw also expires. Thus, conventional ball screws used in driving devices for injection unit and mold-clamping units of injection molding machines were relatively short-lived.
An object of this invention is to extend the life of a ball screw used in a driving device for an injection unit or a mold-clamping unit of an injection molding machine by extending the life of its balls, and thus to improve the performance of the driving device.